As known, in aeronautic turbine engines, the oil used for lubricating the bearings and cooling the transmission tends to mix with air. However, the used oil must be retrieved and re-introduced into the hydraulic circuit of the engine, in order to limit consumption and reduce the polluting substances discharged into the atmosphere. Various devices, either of the static type, named cyclones, or of the rotating type, named rotary separators or deoilers, may be used in order to separate the oil from the air. The latter are generally defined by metallic filtering nets.
Both devices are usually used, to separate oil particles having diameters in very different dimensional ranges. Commonly, cyclone devices are used to separate the larger sized oil drops suspended in air and degassing the larger air bubbles suspended in oil from the oil. Cyclone technology, in all cases, cannot separate the smaller oil drops and the smaller air bubbles. In general, the smaller air bubbles dissolved in the oil do not generate major drawbacks. The smaller oil drops, on the other hand, are separated and collected by means of a rotary separator arranged after the cyclone device along the air-oil mixture flow path.
The rotary separator comprises a toroidal-shaped filter, which is fitted on a rotor and has a pack of annular bodies each defined by a respective metallic filtering net. The filter, on a face thereof, receives the introduced air-oil mixture, lets through the air towards the rotor axis and withholds the oil particles in the pores of the metallic filtering net.
In the metallic filtering net, the rotation has the double function of:                increasing the number of potential collisions of the oil drops against the filtering elements of the net, determining oil coalescence in form of film, which covers such filtering elements;        centrifuging the oil film towards the outer periphery of the filter so as to retrieve such oil.        
Using a rotary separator causes an additional pressure loss in the compressed air system of the turbine engine. Such an aspect becomes particularly critical when the revolution speed of the engine, and thus of the compressor, is relatively low. Indeed, in such operating conditions (idle or taxi conditions) the air pressure in the bearing seals will also be low, with consequent incapacity to maintain sealing if the metallic filter net causes an excessive back pressure. The metallic filtering net structure must be designed in extremely careful manner to obtain a correct trade-off between pressure loss and separation efficiency.
Patent application having publication number EP2156941A1, in the name of the same applicant, and other documents of the prior art teach to manufacture filtering bodies by means of layer by layer or additive manufacturing techniques, which employ an energy beam, i.e. a focused electron beam or a focused laser light beam, to obtain the localized melting and/or sintering of subsequent layers of powders having the same composition as the end product to be obtained. The zones to be melted are established by means of a three-dimensional numerical model which represents the product to be made and which is stored in an electronic unit configured so as to control the energy beam.
These techniques are known, for example, as Direct Laser Forming (DLF), Laser Engineered Net Shaping (LENS), Direct Metal Laser Sintering (DMLS), Selective Laser Melting (SLM), or Electron Beam Melting (EBM).
In the techniques in which sintering is required, the energy beam heats the outer surface of the powder grains so as to melt only such outer surface which joins with that of the adjacent grains. In this manner, the pores of the filter are defined by the gaps between the powder grains joined to one another.
Patent application EP2156941A1, on the other hand, relates to a technique requiring the melting of the powders: the powders have smaller granulometry than those used for sintering and their grains are completely melted. The pores of the filter are defined by the powder parts which are not concerned by the energy beam. Thus, the three-dimensional numerical model represents not only the outer shape of the filtering net but also its inner porous structure.
In particular, the three-dimensional model is generated by defining a base module, which represents a cell of the filtering net, and by replicating the same base module again and again until the shape and dimensions corresponding, in the three-dimensional model, to those of the filtering body to be made are reached. Document EP2156941A1 indicates making a porous cell structure of the diamond structure kind or honeycomb kind.
Thus, the method described in EP2156941A1 allows to define the geometry of the metallic filtering net to obtain the desired porosity of the filter in relatively accurate manner, also as a function of the different zones of the filter and as a function of the pressure loss caused by the rotary separator as a whole. Furthermore, it allows to make the porous structure of the filtering net uniform, and thus to balance the effects of the centrifuge force and position the centre of gravity of the filter exactly on the rotor axis.
Additionally, EP2156941A1 teaches to provide three-dimensional numeral models which integrate, together with the filtering net, solid material elements arranged along the outer edges of the filter, so as to have structure element which support the filtering net and thus reinforce the filter.
The need is felt to improve the known solutions described above, in order to maximize the oil capturing efficacy and to limit, at the same time, the back pressure in the compressed air system of the turbine engine.